The present invention relates to an apparatus for separating undesired particles from a suspension flow of cellulosic fibrous material and for dividing the suspension flow into at least two fractions.
When preparing cellulosic pulp, impurities are supplied both from the outside and also due to deficiencies in the manufacturing process per se. These undesired particles may vary in size and consist of sand, gravel, stones, nuts and bolts, bits of welding electrodes and pieces of metal. It is therefore necessary to remove such undesired particles, particularly from suspensions which either are to be processed further in sensitive machines and equipment which might be damaged by such solid particles or from which the impurities shall be removed as far as possible for other reasons. Various types of equipment, known as scrap separators, for separating out undesired solid particles from a cellulosic pulp have already been proposed, see for instance the patent specifications U.S. Pat. No. 4,737,274 (corresponding to SE 8503372-8), SE 8702744-7 and U.S. Pat. No. 4,941,970 (corresponding to SE 8702745-4). The two latter patent specifications describe screening apparatus with rotating separating means of the same type as that described in U.S. Pat. No. 4,737,274, and with screening drums, for producing accept suitable for bleaching.
U.S. Pat. No. 4,303,508 (corresponding to SE 7903032-6) describes a screening device for separating undesired particles from a suspension flow. The screening device has a rotating screening drum provided with a plurality of circular slits through which a finer portion can pass for separate removal, while a coarser portion containing undesired particles is removed through a reject outlet. The undesired particles can be removed from the coarser portion after passage through the reject outlet. In order to keep the slits of the screening drum open they must be continuously cleared by means of special stationary ridges extending into the slits. The screening device can be used as a partial flow screen to recover a partial flow which is free from twigs, e.g. 25% of a pulp flow can be withdrawn in form of a partial flow free from twigs for liner manufacture.
Two different sets of apparatus with separate drive means have consequently been required to effect partial flow screening and separation of undesired particles from a pulp suspension.